Oxidative simulation of self-heating mechanisms in coal wastes with low-rank organic matter

Self-heating in coal waste dumps is a process depending on variable factors; thus, laboratory simulations are required to find their extent and importance in coal waste alterations. Controlled and known conditions allow us to assess the impact of oxidation on coal waste properties.

Low-rank coal wastes from the Janina Mine (sub-bituminous coal, Upper Silesian coal basin, Poland) were the object of oxidation simulation, performed using the Hastelloy C-276 1-liter reactors (Parr Instrument Co.). For this purpose, ca. 40 and 150g (JAN-1) and ca. 15 and 27g (JAN-2) of the gravel-size (0.5-2.0 cm) coal wastes were heated to 250 and 400°C for 72h in the air atmosphere (synthetic, Air Liquide S.A.). Each experiment was conducted twice using different sample weights. The air-to-rock proportions were established based on stoichiometric calculations taking into account the amount of oxygen in the reactor equivalent to half of the requirement to burnout of organic carbon (from ca. 7 to 38 bar). Gaseous and solid products were collected after the experiments were completed. When the JAN-2 sample was heated to 250°C, the organic matter ignited (at 250°C), resulting in dust emissions and plugging of the manometer connector, which made it impossible to take a gas sample.

Samples of coal wastes oxidized in the experiment were embedded in epoxy resin and prepared for microscope observations in reflected white light and fluorescence using a Carl Zeiss AxioImager.A2m microscope equipped with a 50× oil objective.

Organic matter in coal wastes was dominated by vitrinite, with inertinite and liptinite occurring in lower amounts. The impact of oxidation was mostly seen in vitrinite particles that had paler in colour oxidation rims. Smaller particles and thin lamina of vitrinite were entirely paler in colour and had higher reflectance. That was more seen in mudstone samples containing lower amounts of organic matter compared to claystone. On the other hand oxidation rims were common in claystone as vitrinite particles were thicker.

Iron released from pyrite leads to the formation of oxides and the red color of some of the samples. Contents of illite-smectite and amorphous phases increased during the experiments, while kaolinite was a mineral that was the most affected by the ignition, and its content decreased significantly. 

Larger heterogenous particles of mudstone containing thicker bands of organic matter showed different impacts of temperature. Some liptinite particles were devolatilised but their colour and fluorescence were little changed; others were paler in colour. Vitrinite was plasticised and often had a paler colour. In other parts of the sample organic matter was absent or present as small cracked vitrinite particles and very thin lamina of collotelinite.

The lower grain size of coal wastes causes more extensive alteration due to a higher surface/volume ratio that leads to better exposure to high temperatures. Mudstone samples were more affected, showing paler particles, whereas claystone samples did not show such an effect. The phenomenon at 250°C is much less pronounced. The research shows the significance of laboratory simulation in the investigation of such chaotic and multi-factored processes as self-heating.